Example embodiments of the inventive concepts relate to storage devices, and more particularly, to methods of accessing storage devices.
Semiconductor memory devices may be classified into volatile semiconductor memory devices and non-volatile semiconductor memory devices. A volatile semiconductor memory device may have a fast read and write speed, but may lose stored data once an external power supply is cut off. On the contrary, a non-volatile semiconductor memory device may retain stored data even when an external power supply is cut off. Therefore, a non-volatile semiconductor memory device may be used to store data that should remain regardless of power supply.
A flash memory is a non-volatile memory that may electrically erase data of multiple cells at once and thus may be extensively used in computers and memory cards. Flash memory may be classified into NOR type and NAND type according to a connection state of a cell and a bit line. In NOR flash memory, more than two cell transistors may be connected in parallel to one bit line, a channel hot electrode method may be used to store data, and a Fowler-Nordheim (F-N) tunneling method may be used to erase data. In NAND flash memory, more than two cell transistors may be connected in series to one bit line and a F-N tunneling method may be used to store and erase data.
Each memory cell of a flash memory device may store 1-bit data or multi-bit data. If 1-bit data is stored in one memory cell, a memory cell may have two threshold voltage states (e.g., threshold voltages corresponding to a data 1 and a data 0). If 2-bit data is stored in one memory cell, a memory cell may have threshold voltages corresponding to each of four data states. If 3-bit data is stored in one memory cell, a memory cell may have threshold voltages corresponding to each of eight data states. Recently, a variety of techniques for storing more than 4-bit data is actively under research.